Field of the Invention
The present invention relates to a measurement technique for determining a width of a structure on a mask.
Because of its higher resolution, increasing use is being made of the scanning electron microscope (SEM) in mask inspection in response to the increasing demands placed on the observation of the ever reducing structural dimensions on photomasks. In this case, 3-"sgr"-values of three nanometers are currently required for the reproducibility of structural width measurements in the case of these microscopes. Such low values for system-induced errors are required in order to be able to detect unambiguously production-induced deviations of the structural widths from prescribed specifications of, currently, approximately 20 nanometers. It is even expected for 2002 that deviations of only 10 nanometers can be determined. The specifications can scarcely yet be achieved by current light-optical microscopes.
In order to observe such low 3-"sgr"-values in the case of scanning electron microscopes, it is necessary, in particular, to avoid inaccuracies in focusing. By feeding current into a magnetic lens, there is built up via a coil a magnetic field that focuses an electron beam, and acts as an objective and thereby reduces the electron beam at a distance denoted as a focus to a diameter of only 1-10 nm. Ideally, the surface, which is to be investigated, of the photomask is located at exactly this distance. Deviations from this distance are denoted as defocus. In the scanning electron microscope, the surface of the photomask is scanned by the electron beam by deflecting the electron beam line by line, the excited Auger electrons, which fall below the beam diameter, secondary electrons and, if appropriate, back-scattered electrons being picked up with the aid of a detector. Through shading effects, for example, the Auger electrons and secondary electrons, which amount to only a few electron volts, permit inferences relating to the surface topography of the photomask, or they also supply information on the firstxe2x80x94viewed laterallyxe2x80x94nanometers of the depth of the element structure, and at below the surface. Via a signal amplifier, the detected signal can, when plotted against the object scanning, be structured to form a substantially magnified image of the surface of the photomask. Any possible defocusing of the electron beam effects an expansion of the beam at the surface of the photomask, and thus a lower resolution of the individual surface structures.
Scanning electron microscopes used for mask inspection therefore use a multiplicity of focusing algorithms before the actual measurement of the respective structural width is undertaken. In a known method, a Fourier spectrum of the image determined is evaluated and a resolution-induced threshold frequency is used to determine the quality of the focus, this being followed by a focus correction with a new pick-up of the Fourier spectrum. The best focus is determined with the aid of an optimization method. Unfortunately, it is a common feature of all these focusing methods that the reproducibility of individual measurements is comparatively low, and this is caused, inter alia, by local spatial or temporal changes during measurement. This disadvantageously increases the three 3-"sgr"-values for the measurements of structural widths on photomasks, as a result of which a value, usually not to be exceeded, for this purpose, of 25% of the prescribed tolerance range for structural widths (critical dimension) is in many cases exceeded.
It is accordingly an object of the invention to provide a measurement technique for determining the width of a structure on a mask which overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which the scattering of measured values, is reduced, in structural width determinations repeatedly carried out by scanning electron microscopy such that it is possible to measure smaller structural widths.
With the foregoing and other objects in view there is provided, in accordance with the invention, a measurement technique for determining a width of a structure on a photomask. The method includes the steps of:
a) bringing the photomask with the structure into an object chamber of a scanning electron microscope;
b) adjusting a first focus value of a magnetic lens of the scanning electron microscope;
c) measuring a first width of the structure;
d) measuring a first value for a further parameter characterizing the structure;
e) adjusting a second focus value of the magnetic lens of the scanning electron microscope;
f) measuring a second width of the structure;
g) measuring a second value for the further parameter characterizing the structure;
h) adjusting a third focus value of the magnetic lens of the scanning electron microscope;
i) measuring a third width of the structure;
j) measuring a third value for the further parameter characterizing the structure;
k) adapting a first function with exactly one extreme value to the first, second and third values for the further parameter as a function of the first, second and third focuses, respectively, such that deviations of the first, second and third values from the first function are minimized;
l) determining a focus value assigned to the extreme value of the first function as a best focus;
m) adapting a second mathematical function with exactly one extreme value to the first, second and third widths of the structure as a function of the first, second and third focuses, respectively, such that deviations of the first, second and third widths from the second function are minimized; and
n) determining the width of the structure in relation to a functional value of the second mathematical function which is assigned to the best focus.
The method proposed in accordance with the present invention is already known in a similar way from light-optical microscopes. In the case of the light-optical microscopes, the focus is adjusted as a rule mechanically or by piezoelectric effects by varying the distance between the wafer stage holding the mask and the microscope objective. In accordance with the present invention in the case of the scanning electron microscope the magnetic lens, which acts as the objective, is directly influenced preferably automatically by a control unit in such a way that the magnetic field focusing the electron beam changes due to a change in the controlled current flow for the purpose of adjusting various focusing strengths. The focus therefore does not, as in the light-optical case, describe the distance between the wafer stage and the objective, but the distance, determined by the strength of the magnetic field, of the strongest focusing of the beam from the magnetic lens. Ideally, the current flow in the coil generating the magnetic lens will generate a magnetic field such that the focus thus generated in the scanning electron microscope is equal to the distance of the surface of the photomask from the magnetic lens.
The invention is based on the finding that individual measurements cannot be reproduced by using different focusing algorithms. If a CD measurement is repeated by using a value for the focus adjusted with the aid of conventional algorithms, a scattering of the measured CD values results despite a constant focal value. The present invention circumvents this problem by virtue of the fact that the best focus is not fixed from the very start, but that use is made of further parameters that characterize the structure under consideration in an entirely local fashionxe2x80x94for example, the contrast or the slope of the structural edges in this case. The contrast describes the difference in intensity between the maximum measured value of the structure itself and the minimum measured value of the surroundings. The slope results from the intensity gradient at the location of the edge. The locally determined parameters directly supply the quality of the adjusted focus, and are therefore used in accordance with the present technique in order to determine the focus. Since a scatter can also be present here, the characteristic of the relevant measured parameter against the adjusted focal value is fitted by a mathematical function. The extreme value of such a fitted function reflects the best adjusted focal value.
In this document, the parameters characterizing the structure are denoted as those that permit the focus to be inferred directly because of their local validity. In addition to those mentioned, the designation therefore also includes combinations of the parameters such as structural contrast and slope, but also further parameters not illuminated here. It would also be possible to use the material contrast as a parameter.
In addition to the parameters, such as contrast or slope, obtained directly from the images taken with the scanning electron microscope, according to the invention the structural widths, that is to say the CD values, are also measured indirectly through the distance of the structural edges. As a function of the adjusted focal value, the CD values form a scattered distribution to which a mathematical function is fitted. Depending on whether the structure is bright or dark by comparison with the surroundings, the maximum or minimum of the function deviates as a rule in the argument, that is to say the focal value, from that of the best focus determined using the other parameters such as contrast or slope. Specifically, the maximum or minimum structural width measured for the focal value does not necessarily reflect an optimally adjusted focus. In some circumstances, however, only one variation of the distances of the edge profiles is found here as a function of the beam diameter or the focus. Smeared edge profiles owing to a poorly adjusted focus can supply maximum or minimum distances from one another, that is to say structural widths. The present technique precludes this by using the best focus obtained from the further parameter as an argument of the function obtained for the CD value, on which a statistically very effectively reproducible structural width can advantageously be read off or obtained.
Consequently, the system error (3"sgr") in the case of the scanning electron microscopes is substantially reduced for the measurement of structural widths, and it becomes possible to measure structural widths with tolerances of 10 nm. In the case of scanning electron microscopes, there arises, moreover, the decisive advantage that no mechanical correction need be carried out, nor is there any need for prefocusing. Once the image has been taken for a focus, the next focal value can be adjusted immediately by changing the coil current in the magnetic lens. Only a slight extra outlay on time results therefrom.
In accordance with an added mode of the invention, there is the step of using a structure contrast or a slope of a structural edge as the further parameter characterizing the structure.
In accordance with an additional mode of the invention, there is the step of using parabolas in each case for the first function and the second mathematical function.
In accordance with another mode of the invention, there is the step of calculating a sum of mean square deviations for the minimization steps.
In accordance with a further mode of the invention, there is the step of regulating a current flow in the magnetic lens of the electron microscope to adjust at least one of the first, second, and third focus values.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a measurement technique for determining the width of a structure on a mask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.